Hey there, At the end of the day it is up to you about this situation. I know you are probably not going to want to hear this but i think you should accept his choice and move on.
Your ex has child now, to be honest he has different priorities with his child. I don't think you should try and get back with him mainly due to the sake of the baby. You would not want to break the pair of them up. It would not be fair on his girlfriend or the baby.
Speak to your ex about what the future may hold, because it is down to him. If he likes you then good for you,but bearing in mind he would have to stick by his child would you be willing for him to be like that? He would have to what is right by the child.
Maybe the two of you broke up for a reason, i know it seems hard but there are other people out there, who maybe more right for you. Trust me he will come along eventually.
Good Luck!